Characterization of antibiotic determinants and heavy metal resistance genes in <i>Escherichia coli</i> from pigs in Catalonia.

More antibiotics are administered to livestock animals than to treat human infections. Industrialization, large animal densities and early weaning mean pigs are exposed to more antibiotics than any other livestock animal. Consequently, antimicrobial resistance (AMR) is common among commensal and pathogenic bacteria. Heavy metals (HMs) are also often used as feed additives for growth promotion and infection prevention alongside antimicrobials, and increased exposure to copper, zinc and cadmium can further encourage AMR through co-selection. In this study, we sequenced an archived collection of 112 Escherichia coli isolates from pigs in Catalonia using short- and long-read sequencing methods to detect AMR and HM tolerance genes. The most common AMR genes were mdfA (84.8%), aph(3″)-Ib (52.7%), bla _TEM-1B (45.6%) and aph(6)-Id (45.6%). Genes relevant to public health, such as the extended-spectrum β-lactamases (15.4%), bla _CTX-M type or bla _SHV, or mobile colistin resistance (mcr) genes (13.4%), such as mcr-1, were also found. HM tolerance genes were present in almost every genome but were rarely located in plasmids, and, in most cases, AMR and HM tolerance genes were not located on the same plasmids. Of the genes predicted to increase tolerance to HMs, only those with activity to mercury were co-located on plasmids alongside other AMR determinants. However, mercury is rarely used in pig farming and does not support a scenario where AMR and HM genes are co-selected. Finally, we identified the exclusive association between mcr-4 and ColE10 plasmid, which may help target interventions to curtail its spread among pig Escherichia coli.